The present invention relates to a system for transmitting the power of an engine to four driving wheels of a four-wheel drive vehicle.
In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformly distribute the torque of an engine to the front wheels and to the rear wheels of the vehicle.
However, if one of the four wheels skids, the torque is not transmitted to the four wheels, so that the vehicle stops. In order to prevent such a problem, a lock mechanism for locking the central differential is provided.
As a lock mechanism for the system, a system employing a solenoid-operated dog clutch is known. Further, Japanese Patent Application Laid-Open No. 59-179425 discloses a system in which the front wheels are connected to the rear wheels by a fluid clutch or a viscous clutch. When the rotational speeds of the rear wheels are lower than a predetermined speed range compared with those of the front wheels, the rear wheels are driven by the power for the front wheel drive through a one-way clutch.
However in the former case, since the clutch is operated by a driver in accordance with conditions of the surface of a road, it causes troublesome operation.
In the latter system, it is difficult to determine the capacity of the viscous clutch for setting a viscous mode and a hump mode. Further, in order to obtain an appropriate operational characteristic, the viscous clutch must have a comparatively large capacity.